An air-fuel ratio sensor that senses the air-fuel ratio of the exhaust gas is provided in the exhaust system of an internal combustion engine in some cases in order to control the air-fuel ratio of the internal combustion engine. As an air-fuel ratio sensor, a limiting current type air-fuel ratio sensor has been developed as described in Patent Document 1. This limiting current type air-fuel ratio sensor comprises a sensor element made of an oxygen ion conductive solid electrolyte such as zirconia, a pair of platinum electrodes provided on two sides of the sensor element, and a porous diffusion-controlling layer (which is also referred to as a “diffusion resistance layer” in some cases) provided on the element in such a way as to cover one of the electrodes. The diffusion-controlling layer controls or limits the diffusion of the exhaust gas toward the sensor element.
In such a limiting current type air-fuel ratio sensor, when the exhaust gas having passed through the diffusion-controlling layer comes in contact with one electrode while a predetermined voltage is applied between the electrodes, oxygen ions are pumped between the electrodes through the sensor element, whereby a current flows between the electrodes. In connection with this, since the diffusion of the exhaust gas toward one electrode is limited by the diffusion-controlling layer, there is a region in which the current saturates to a constant value in spite of increases in the applied voltage. This current value (or the limit current value) has a characteristic proportional to the oxygen concentration and the air-fuel ratio of the exhaust gas, and therefore the air-fuel ratio can be determined by measuring the limit current value.
The reason why a constant current of the limit current value correlating with the air-fuel ratio of the exhaust gas flows between the electrodes when the predetermined voltage is applied between the electrodes is that the current is diffusion-limited as the exhaust gas passes through the diffusion-controlling layer. However, since the molecular mass of the hydrogen (H2) component contained in the exhaust gas is smaller than those of the other components such as oxygen (O2), the hydrogen component passes through the diffusion-controlling layer at a higher diffusion rate characteristically. In consequence, if the amount of H2, which serves as a reducing agent, reaching one electrode is larger than that of O2, which serves as an oxidizing agent, the reducing agent becomes overabundant in the vicinity of this electrode, whereby an air-fuel ratio that is erroneously richer than its actual value may be obtained as the measurement result.
This is addressed by the technology disclosed in Patent Document 1 in which the surface of the diffusion-controlling layer is covered with a catalyst layer having an oxidizing ability to oxidize H2 contained in the exhaust gas before the exhaust gas passes through the diffusion-controlling layer, whereby the accuracy in measuring the air-fuel ratio is improved.
Patent Document 2 also discloses a technology in which a catalyst layer with which at least one electrode exposed to the exhaust gas is covered is provided, and a mask layer through which the exhaust gas cannot pass is provided on a portion on which the catalyst layer is not provided.    Patent Document 1: Japanese Patent Application Laid-Open No. 11-237361    Patent Document 2: Japanese Patent Application Laid-Open No. 2003-202316    Patent Document 3: Japanese Patent Application Laid-Open No. 2004-316498    Patent Document 4: Japanese Patent Application Laid-Open No. 2006-337205